DC DC Converters Transient Suppression

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  • Application Note - Interpoint

    Crane Aerospace & Electronics Power Solutions

    Transient Suppression:Switching Power Supplies

  • Power line transient requirements are a subject which should be addressed at the beginning of a project, not in the later stages. If let go till later, there may not be enough room left over for a good solution. When the time comes, two very important questions should be asked:

    1) What is the transient source impedance? Inquire about both the real & imaginary parts.

    2) What is the volt x second product of the transient?

    If the answer is a large source impedance and a small volt x second product, the problem will be simple to solve. If the volt x second product is large and the source impedance low, the usual case, the solution will be more difficult.

    The following is written around a 28 volt DC aircraft power bus, where positive transients should be contained within the 35 to 50 volt area, where the maximum long term transient rating for most Interpoint power converters is 50 volts. For power converters which have an 80 volt long term transient rating, suppression to within less than say 70 volts should be safe, but the output response of the power supply may require a smaller limit.

    The means which are available to suppress voltage transients are listed in the following:

    1) Transorbs, MOVs. Brute force approach.2) Shunt capacitors. Will work for high source Zs. 3) Series inductor to limit DI/DT.4) Buck regulator for lower voltage longer term transients.5) Stripper, a series regulator which opens to take the transient

    and maintain like a 40 V input to the power supplies for the transient duration.

    6) Combinations of 1, 2, and 3 above.

    Figure 1 shows the functional schematic of a switching power supply with some of the various means of transient suppression shown at its input. The power supply has an incrementally nega-tive input impedance which must be considered before inserting any suppression network in series with its input. The negative input Z is due to the constant power input, at constant load, as a function line voltage. Simply stated, (Vin x Iin) = K, a constant at constant load, and Iin = (K/Vin). The decreasing input current with an increasing input voltage defines a hyperbola, the slope at the operating point being the negative Z. The input current is largest at minimum line voltage, and the input Z minimum at this point. For stable operation, the impedance looking back into any tran-

    INPUT

    600 kHzCONTROLLER

    MAG. COUPLER

    +VOUT

    COMREFREF AMP

    LIMITER

    ZS = ?

    VIN

    CASE

    +

    SYN

    INH

    RTN

    TRANSIENT SOURCE

    MTR SERIES SINGLE OUTPUT TYPESCHEMATIC DIAGRAM

    VOLT x SECOND PRODUCT ?

    OR

    BUCK REGULATOR

    ORSTRIPPER

    Figure 1: TransienT suppression For a swiTching power supply

    Crane Aerospace & Electronics Power Solutions

    Transient Suppression: Switching Power Supplies

    Crane Aerospace & ElectronicsCrane Electronics Group (Interpoint Brand)PO Box 97005 Redmond WA 98073-9705425.882.3100 electronics@craneae.comwww.craneae.com

    Page 1 of 12Rev C - 20061206

    APPlicATion noTeAlthough the concepts stated are universal, this application note was written specifically for Interpoint products.

  • sient suppression network must be less than that seen looking into the switching power supply at minimum line voltage, and over the control loop bandwidth. It may be advisable to consult one of the Interpoint applications engineers before finalizing any suppression network design. Also, refer to Figure 2 which has some additional information.

    SHORT TERM TRANSIENTSFigure 3 shows some typical transient waveforms taken from DO-160C and MIL-STD-461C. The top waveform is an approxi-mated damped half sine with a peak of 600 volts and a half

    period of 10 s. The 50 ohm source impedance allows it to be suppressed with a shunt capacitor or transorb across the 28 volt line. In either case the peak suppression current will not exceed 12 amps, 600 V/50 ohms. Using a 600 V rectangular pulse for simplicity, a 10 f low ESR cap placed in shunt with the line will reduce the peak transient response to about 12 volts on top of the 28 volt power line. The time constant of the RC will be 50 x the transient duration, hence the response is 2% of the transient plus the drop across the ESR. Refer to Figure 4. A transorb will also work in place of or with the capacitor. A suitable surface mount device is SMCJ30A from Microsemi Corp. This device should clamp the line at about 40 volts or below and also has a

    PWM SWITCHING POWER SUPPLY ZIN

    600 kHzCONTROLLER

    MAG. CASE

    INH

    +VIN +VOUT

    COMREFREF AMP

    LIMITER

    RTN

    PO = VO2

    / RL

    RLLOAD

    V IN

    IIN

    V X I = K ZIN =

    V X I = K IS A HYPERBOLA

    NORMAL RES.R = K = V/I

    ZIN = VIN2 / PINPIN = PO /Eff.VIN = ; =

    ASSUMES PO & PIN = K1ZIN = , & IS NEG.

    PINIIN2

    dVINdIIN

    PINIIN

    KIIN2

    ZIN

    SYNC

    INPUT FILTER

    RD = DAMPING

    JL

    ZIN

    fOSC LOG FREQUENCY

    Z -

    INPUT FILTER

    600 kHzCONTROLLER

    MAG. COUPLER

    +VOUT

    COMREFREF AMP

    LIMITER

    PO = VO2 / RL

    RLLOAD

    R

    ZS =

    VS = LOW ZSUPPLY

    USE SERIES DAMPINGNETWORK OF C AND RTO STABILIZE SYSTEM.

    VIN

    ZIN

    SYNCINH

    RTN

    CASE

    zs

    D

    RD = LC FOR Q=1.0fRES

    D

    PWM SWITCHING POWER SUPPLY STABILITY EXAMPLE

    Figure 2: negaTive Zin and damping For sTabiliTy

    Crane Aerospace & Electronics Power Solutions

    Transient Suppression: Switching Power Supplies

    www.craneae.com Page 2 of 12Rev C - 20061206

    APPlicATion noTe

  • forward rating of 200 Amps for 1/120 of a second which would be useful for suppressing negative transients, if any. Alternately, a forward rectifier in series with the positive line can be used for this purpose.

    The MIL-STD-461C transient of Figure 3 has a low source Z and will be harder to suppress than the previous case. Unless defined otherwise, the source Z should be assumed to be zero. A

    value of 0.5 ohms is often allowed for testing. For the purpose of analysis, a value of 0.1 ohms is used.

    The 0.15 s 200 volt rectangular transient could be suppressed with a low ESR ceramic capacitor, or a transorb. The transient line current, however, would be very large. Adding a small series inductor will reduce the current to a manageable level. Since the transient will substantially appear across the inductor, the rate at

    2 s MAXIMUM

    10 s

    MAXIMUM

    TYPICAL FOLLOW - ON CURVE

    600

    MINIMUM OPEN CIRCUIT VOLTAGE

    THE WAVEFORM SOURCE IMPEDANCE SHALL BE 50 ; THE SPECIFIED VOLTAGE AND DURATIONS AREFOR OPEN CIRCUIT CONDITIONS ONLY. THE PEAK VOLTAGE MAY BE SUBSTANTIALLY LOWER WITH THE EQUIPMENT CONNECTED. THE TESTER SOURCE IMPEDANCE CAN BE VERIFIED BY TESTING WITH A 50 LOAD RESISTOR AND SHOULD PRODUCE ONE HALF OF THE SPECIFIED VOLTAGE 10%

    DO - 160C - FIGURE 17-1 CATEGORY A VOLTAGE SPIKE WAVEFORM

    E ( )0.5E

    Time

    t ( )

    ( )E 200 V

    TimeVolts

    Volts

    NOTE: The test sample shall be subjected to the spike(s) with the waveform shown and with the specified voltage(s) and pulsewidth(s).

    MIL-STD-461C- ACCEPTABLE WAVESHAPES FOR CSO6

    t ( )

    t = 0.15 sec1t 2 = 10.0 sec

    Figure 3: Typical TransienTs do-160c and mil-sTd-461c

    Crane Aerospace & Electronics Power Solutions

    Transient Suppression: Switching Power Supplies

    www.craneae.com Page 3 of 12Rev C - 20061206

    APPlicATion noTe

  • Tran

    sient

    Res

    pons

    e (V

    out)

    0.00

    10.00

    20.00

    30.00

    40.00

    50.00

    5100 5200 5300 5400 5500

    Tran

    sient

    Volt

    age

    - KV in

    0.00

    .13

    .26

    .39

    .52

    .65

    50 600 V 10 sec Transients

    Time (s)

    +10 F600 V

    28 V 0.1

    TRANSIENTRESPONSEVOLTAGE

    50 ZO

    Figure 4: TransienT response

    which the current rises will be V/L, where V is the transient peak voltage L is the inductance in henries, and the rate is in Amps/second. If we limit the peak current to 10 Amps, the inductance = (V)(t)/(10) or greater, where t is the pulse duration of 0.15 s. The calculated inductance is then 3 henries. Either a shunt capacitor or transorb can be used for the small value inductor to work against. In this case we will use a 10 fd capacitor forming a resonant circuit with the inductor. Based on a Q = 1, the total damping resistance in series with the L and C should be = (L/C) = 0.55 ohms to minimize the driving point impedance. A 0.5 ohm resistor is added in series with the 10 f capacitor, and includes its ESR. The circuit models and results are shown on Figures 5 and 6. Note that the output response to the 200 volt transient rises to only about 34 volts.

    The inductor has to carry not only the transient current, but also the normal load current. A sample design on a 0.4 diameter Kool Mu toroid will carry 20 amps at a flux density of about 80% of saturation. The inductor has 7 turns of #22 AWG wire with a DC

    resistance of about 0.006 ohms at 23* C. The part number of the Mag. Inc. core is 77040-A7. The formulas for inductance and flux density are given below for those who want to try it themselves.L = 4pAN2 x 10-7, where A = the cross sectional area in square meters, = the magnetic path length in meters,u = permeability,N = number of turnsI = Maximum inductor current - Amps,B = 4pNI x 10-3The units of inductance are Henries, and hose of Flux